1. Field of the Invention
The present invention relates to a sensor used to detect a printing sheet in a image forming machine, especially to technology for preventing malfunction of the sensor covered by foreign material such as paper powder or dust.
2. Description of the Related Art
Generally, in an image forming machine such as a stencil printing machine, printing sheets are stacked on a predetermined position, and the sheets are picked up one by one and fed into an image forming section. Then, the sheet is discharged therefrom after an image is formed on the sheet. In such image forming machine, a sensor may be situated for obtaining a detection signal of the printing sheet to control timing of printing operation or to detect jamming of the printing sheet during printing operation.
For example, the printing sheet picked up from the predetermined position is conveyed by rollers through a conveying path composed of guiding plates, and then the sheet is detected by a light sensor of non contact type at a leading end thereof, so that it is judged whether conveying timing of the printing sheet is appropriate.
During the conveyance of the printing sheet by the rollers through the conveying path made of the guiding plates, the printing sheet produces much foreign material while being rubbed against edges of holes where the rollers are situated. Accordingly, as shown in FIG. 6(a), paper powder I accumulates on the light sensor of the non-contact type, thereby deteriorating detecting function of the sensor. To prevent this, a transparent cover may be attached onto the light sensor of the non-contact type; however, this is not actually adopted due to troublesome assembly operation needed by the complicated structure.
Further, as shown in FIG. 6(b), a light sensor of reflective type may be arranged downward above the conveying path to reduce accumulation of the foreign material. This structure requires periodic cleaning of a bottom surface of the sensor since the paper powder inevitably adheres thereto in a long term. Further, the problem arises from not only the paper powder, but also dust accumulated in a long term.
An object of the present invention is to simply and periodically clean a sensor that tends to be covered by paper powder or dust in an image forming machine having the sensor to detect a printing sheet.